1. Field of the Invention
The present invention relates to a mask forming method for forming a mask used when dry etching an etched body, a dry etching method for dry etching an etched body using a mask formed in accordance with the mask forming method, a method of manufacturing an information recording medium that manufactures an information recording medium using the dry etching method, and a mask forming functional layer that is formed so as to cover an etched body.
2. Description of the Related Art
As one example of a method of manufacturing an information recording medium that forms a mask using this type of mask forming method, the present applicant has proposed a method of dry etching a magnetic material in Japanese Laid-Open Patent Publication No. 2003-058382. More specifically, in the dry etching method proposed by the present applicant, first a base oriented layer, a magnetic thin-film layer, a first mask layer, a second mask layer, and a resist layer are formed in that order on an Si (silicon) substrate to fabricate a processed body. In this case, the base oriented layer is formed with a thickness of around 30 to 300 nm by sputtering using Cr (chromium), a Cr alloy, CoO (cobalt oxide) or MgO (magnesium oxide), NiO (nickel oxide), or the like. The magnetic thin-film layer is formed with a thickness of around 10 to 30 nm by sputtering using a Co (cobalt) alloy. The first mask layer is formed with a thickness of around 10 to 50 nm by sputtering using Ta (tantalum), and the second mask layer is formed with a thickness of around 10 to 30 nm by sputtering using Ni (nickel). In addition, the resist layer is formed with a thickness of around 30 to 300 nm by spin coating using a positive-type resist.
Next, a convex/concave pattern is formed on the second mask layer by carrying out an exposing process on the resist layer of the processed body using an electron beam exposing apparatus and then carrying out a developing process. Next, ion beam etching with Ar (argon) gas is carried out on the second mask layer using the resist layer in which the convex/concave pattern has been formed as a mask to form a convex/concave pattern on the first mask layer. Next, reactive ion etching with CF4 gas or SF6 gas is carried out on the first mask layer using the second mask layer in which the convex/concave pattern has been formed as a mask to form a convex/concave pattern on the magnetic thin-film layer. After this, reactive ion etching with a mixture of CO gas and NH3 gas is carried out on the magnetic thin-film layer using the first mask layer in which the convex/concave pattern has been formed as a mask to form a convex/concave pattern on the base oriented layer. Next, the first mask layer that is left on the convex patterns in the convex/concave pattern is etched using CF4 gas or SF6 gas. By doing so, the fine machining of the magnetic thin-film layer of the processed body is completed, thereby completing a magnetic information recording medium as an information recording medium.
By investigating the dry etching method described above, the present inventors discovered the following problem to be solved. That is, with the above dry etching method, by carrying out reactive ion etching on the first mask layer (a mask forming functional layer) using the second mask layer (also a mask forming functional layer) in which a convex/concave pattern has been formed, a mask (convex/concave pattern) for use when dry etching the magnetic thin-film layer is formed. In this case, when the first mask layer has been formed by sputtering Ta, a first mask layer with crystal grains of various different magnitudes is normally formed on the magnetic thin-film layer. When reactive ion etching of the first mask layer has been carried out using the second mask layer in which the convex/concave pattern has been formed as a mask, the first mask layer is etched with the crystal grains described above as the units of elimination (also referred to as the “elimination units”). Accordingly, since reactive ion etching is carried out with the various crystal grains as the elimination units when forming a convex/concave pattern in the first mask layer, pattern fluctuations, where the convex patterns become jagged in the width direction corresponding to the sizes of the crystal grains, occur in the convex/concave pattern formed on the magnetic thin-film layer. When reactive ion etching of the magnetic thin-film layer has been carried out using the first mask layer, in which a convex/concave pattern with pattern fluctuations has been formed, as a mask, as shown in FIG. 13, pattern fluctuations also occur in the convex/concave pattern formed in the magnetic thin-film layer. It should be noted that FIG. 13 is a figure-substituting photograph in which the convex/concave pattern of the magnetic thin-film layer has been photographed (as one example, the ratio of the width of the convex patterns to the widths of the concave patterns is 4:1), with outline parts of the convex patterns being shown in white.
In this case, when a discrete track-type magnetic recording medium, for example, has been manufactured by dry etching, depending on the formation pitch of the data recording tracks, there is the risk of it being difficult to record and reproduce recording data properly due to the presence of the pattern fluctuations described above. More specifically, for a discrete track-type magnetic recording medium that is being developed by the present inventors, to improve the recording density, the width of the non-magnetic parts (concave parts in the convex/concave pattern formed in the magnetic thin-film layer) between the respective data recording tracks is set at 200 nm or below. On the other hand, the size of the crystal grains described above are comparatively large at 25 nm to 35 nm, and due to the presence of these crystal grains, the amount of fluctuation (the width of the fluctuations) for the pattern fluctuations is 25 nm to 35 nm. Since concaves and convexes (pattern fluctuations) of a size that is ¼ to ⅓ of the width of the non-magnetic parts, which is 100 nm for example, are present, there is the risk that proper recording and reproduction will become difficult for the data recording tracks. For this reason, the realization of a means for forming a convex/concave pattern with small pattern fluctuations in the magnetic thin-film layer is desired. It should be noted that the pattern fluctuations do not just occur during reactive ion etching but also occur during various other etching methods (such as ion beam etching).